1. Field of the Invention
This invention relates to zoom optical systems, and more particularly to a zoom optical system providing a practical way to obtain a moderate to large aperture of approximately 6 inches to more than 16 inches in diameter, and high speed on the order of f/2.25 while maintaining broad spectral image correction. The system is suitable for use with electro-optical detectors as well as photographic emulsions and may be used in reverse as a target projector, light transmitter or collimator optical system. A zoom range having a focal length ratio of at least 3:1 is readily obtainable.
2. Description of the Prior Art
Conventional large aperture sensors (telescopes) use either reflective or catadioptric main optics because the residual chromatic aberrations found in conventional all-refractive lens systems provides unacceptable image quality. Zoom systems in large aperture reflective or catadioptric collector systems generally comprise a main objective coupled with a relay optical system. Fixed focused relay optical systems are common, however, the use of zoom optical relays to date has been exceedingly limited because designs providing acceptably small lens element sizes tend to produce excessively large chromatic aberrations. The designs having acceptable color correction, in turn, result in very large element sizes, and in practice, excessive vignetting.
One of the reasons for the above discussed problem is that the prior art systems utilize a "real object relay system" wherein an image is first formed by a main light collecting objective and relayed by a positive power zoom system. A field lens is sometimes placed near the focus of the main objective to reduce element diameter in the relay zoom group. The real object relay system may be considered as three separate groups, namely, a collimator, a zoom virtual object relay and a decollimator or backing group. Such a system results in a preponderance of positive focusing power with the collimator and decollimator both having positive focusing power. The zoom virtual object relay can be made to have zero focusing power for chromatic considerations, and the field lens has little effect on chromatic aberrations. However, the positive focusing power of the collimator and decollimator results in large chromatic aberrations that must be corrected by extensive use of special refractive material, even though such materials may pose additional problems in obtaining high optical speed. An example of a typical real object relay system is disclosed in U.S. Pat. No. 3,152,214. The aperture and speed of a system of the type disclosed in the aforesaid U.S. Pat. No. 3,152,214 would be limited by practical considerations to the order of approximately four inches and f/4, respectively.